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Andrey Baydin, Natsumi Komatsu, Fuyang Tay, Saunab Ghosh, Takuma Makihara, G. Timothy Noe, and Junichiro Kono
Experimental setup for showing giant THz polarization rotation in an aligned CNT film. (a) Schematic of THz transmission and reflection through the CNT film and substrate; (b) THz waveform in the time domain indicating the existence of a second pulse due to reflections in the substrate as shown in (a); (c) experimental configuration showing wire-grid polarizer, the sample, and the schematic of the polarization rotation of the propagating THz pulse; (d) polarization angle 𝜃 defined as the angle between the CNT alignment direction and the polarization of the incident THz electric field.
For easy manipulation of polarization states of light for applications in communications, imaging, and information processing, an efficient mechanism is desired for rotating light polarization with a minimum interaction length. Here, we report giant polarization rotations for terahertz (THz) electromagnetic waves in ultrathin (∼45nm), high-density films of aligned carbon nanotubes. We observed polarization rotations of up to ∼20∘ and ∼110∘ for transmitted and reflected THz pulses, respectively. The amount of polarization rotation was a sensitive function of the angle between the incident THz polarization and the nanotube alignment direction, exhibiting a “magic” angle at which the total rotation through transmission and reflection becomes exactly 90°. Our model quantitatively explains these giant rotations as a result of extremely anisotropic optical constants, demonstrating that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz polarization devices.
Ultrathin, broadband polarization rotators are made possible
by ultrathin carbon nanotube films developed at RiceUniversity
in 2016. The films of highly aligned single-walled nanotubes were first made in
2016. Credit: Kono Laboratory/Rice University
It's always good when your hard work reflects well on you.
With the discovery of the giant polarization rotation of light, that is literally so.
The ultrathin, highly aligned carbon nanotube films first made by Rice University physicist Junichiro Kono and his students a few years ago turned out to have a surprising phenomenon waiting within: An ability to make highly capable terahertz polarization rotation possible.
This rotation doesn't mean the films are spinning. It does mean that polarized light from a laser or other source can now be manipulated in ways that were previously out of reach, making it completely visible or completely opaque with a device that's extremely thin.
The unique optical rotation happens when linearly polarized pulses of light pass through the 45-nanometer film and hit the silicon surface on which it sits. The light bounces between the substrate and film before finally reflecting back, but with its polarization turned by 90 degrees.
This only occurs, Kono said, when the input light's polarization is at a specific angle with respect to the nanotube alignment direction: the "magic angle."
The discovery by lead author Andrey Baydin, a postdoctoral researcher in Kono's lab, is detailed in Optica. The phenomenon, which can be tuned by changing the refractive index of the substrate and the film thickness, could lead to robust, flexible devices that manipulate terahertz waves.
RiceUniversity physicists
have made unique broadband polarization rotators with ultrathin carbon nanotube
films. The films optically rotate polarized light output by 90 degrees, but
only when the input light's polarization is at a specific angle with respect to
the nanotube alignment direction: the "magic angle." Credit: Kono
Laboratory/Rice University
Kono said easy-to-fabricate, ultrathin broadband polarization rotators that stand up to high temperatures will address a fundamental challenge in the development of terahertz optical devices. The bulky devices available until now only enable limited polarization angles, so compact devices with more capability are highly desirable.
Because terahertz radiation easily passes through materials like plastics and cardboard, they could be particularly useful in manufacturing, quality control and process monitoring. They could also be handy in telecommunications systems and for security screening, because many materials have unique spectral signatures in the terahertz range, he said.
"The discovery opens up new possibilities for waveplates," Baydin said. A waveplate alters the polarization of light that travels through it. In devices like terahertz spectrometers used to analyze the molecular composition of materials, being able to adjust polarization up to a full 90 degrees would allow for data gathering at a much finer resolution.
"We found that specifically at far-infrared wavelengths—in other words, in the terahertz frequency range—this anisotropy is nearly perfect," Baydin said. "Basically, there's no attenuation in the perpendicular polarization, and then significant attenuation in the parallel direction.
"We did not look for this," he said. "It was completely a surprise."
He said theoretical analysis showed the effect is entirely due to the nature of the highly aligned nanotube films, which were vanishingly thin but about 2 inches in diameter. The researchers both observed and confirmed this giant polarization rotation with experiments and computer models.
"Usually, people have to use millimeter-thick quartz waveplates in order to rotate terahertz polarization," said Baydin, who joined the Kono lab in late 2019 and found the phenomenon soon after that. "But in our case, the film is just nanometers thick."
"Big and bulky waveplates are fine if you're just using them in a laboratory setting, but for applications, you want a compact device," Kono said. "What Andrey has found makes it possible."
Filchito Bagsican, Michael Wais, Natsumi Komatsu, Weilu Gao, Weilu Gao, Lincoln W. Weber, Kazunori Serita, Hironaru Murakami, Karsten. Held, Frank A. Hegmann, Masayoshi Tonouchi, Junichiro Kono, Iwao Kawaya, Marco Battiato
Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.
Physicists provide direct evidence of entanglement’s role in quantum criticality
In a new study, U.S. and Austrian physicists have observed quantum entanglement among “billions of billions” of flowing electrons in a quantum critical material.
The research, which appears this week in Science, examined the electronic and magnetic behavior of a “strange metal” compound of ytterbium, rhodium and silicon as it both neared and passed through a critical transition at the boundary between two well-studied quantum phases.
The study at Rice University and Vienna University of Technology (TU Wien) provides the strongest direct evidence to date of entanglement’s role in bringing about quantum criticality, said study co-author Qimiao Si of Rice.
“When we think about quantum entanglement, we think about small things,” Si said. “We don’t associate it with macroscopic objects. But at a quantum critical point, things are so collective that we have this chance to see the effects of entanglement, even in a metallic film that contains billions of billions of quantum mechanical objects.”
Si, a theoretical physicist and director of the Rice Center for Quantum Materials (RCQM), has spent more than two decades studying what happens when materials like strange metals and high-temperature superconductors change quantum phases. Better understanding such materials could open the door to new technologies in computing, communications and more.
The international team overcame several challenges to get the result. TU Wien researchers developed a highly complex materials synthesis technique to produce ultrapure films containing one part ytterbium for every two parts rhodium and silicon (YbRh2Si2). At absolute zero temperature, the material undergoes a transition from one quantum phase that forms a magnetic order to another that does not.
Physicist Silke Bühler-Paschen of the Vienna University of Technology (Photo by Luisa Puiu/TU Wien)
At Rice, study co-lead author Xinwei Li, Junichiro Kono, performed terahertz spectroscopy experiments on the films at temperatures as low as 1.4 Kelvin. The terahertz measurements revealed the optical conductivity of the YbRh2Si2 films as they were cooled to a quantum critical point that marked the transition from one quantum phase to another.
then a graduate student in the lab of co-author and RCQM member
“With strange metals, there is an unusual connection between electrical resistance and temperature,” said corresponding author Silke Bühler-Paschen of TU Wien’s Institute for Solid State Physics. “In contrast to simple metals such as copper or gold, this does not seem to be due to the thermal movement of the atoms, but to quantum fluctuations at the absolute zero temperature.”
To measure optical conductivity, Li shined coherent electromagnetic radiation in the terahertz frequency range on top of the films and analyzed the amount of terahertz rays that passed through as a function of frequency and temperature. The experiments revealed “frequency over temperature scaling,” a telltale sign of quantum criticality, the authors said.
Kono, an engineer and physicist in Rice’s Brown School of Engineering, said the measurements were painstaking for Li, who’s now a postdoctoral researcher at the California Institute of Technology. For example, only a fraction of the terahertz radiation shined onto the sample passed through to the detector, and the important measurement was how much that fraction rose or fell at different temperatures.
Former Rice University graduate student Xinwei Li in 2016 with the terahertz spectrometer he later used to measure entanglement in the conduction electrons flowing through a “strange metal” compound of ytterbium, rhodium and silicon. (Photo by Jeff Fitlow/Rice University)
“Less than 0.1% of the total terahertz radiation was transmitted, and the signal, which was the variation of conductivity as a function of frequency, was a further few percent of that,” Kono said. “It took many hours to take reliable data at each temperature to average over many, many measurements, and it was necessary to take data at many, many temperatures to prove the existence of scaling.
“Xinwei was very, very patient and persistent,” Kono said. “In addition, he carefully processed the huge amounts of data he collected to unfold the scaling law, which was really fascinating to me.”
Making the films was even more challenging. To grow them thin enough to pass terahertz rays, the TU Wien team developed a unique molecular beam epitaxy system and an elaborate growth procedure. Ytterbium, rhodium and silicon were simultaneously evaporated from separate sources in the exact 1-2-2 ratio. Because of the high energy needed to evaporate rhodium and silicon, the system required a custom-made ultrahigh vacuum chamber with two electron-beam evaporators.
“Our wild card was finding the perfect substrate: germanium,” said TU Wien graduate student Lukas Prochaska, a study co-lead author. The germanium was transparent to terahertz, and had “certain atomic distances (that were) practically identical to those between the ytterbium atoms in YbRh2Si2, which explains the excellent quality of the films,” he said.
Si recalled discussing the experiment with Bühler-Paschen more than 15 years ago when they were exploring the means to test a new class of quantum critical point. The hallmark of the quantum critical point that they were advancing with co-workers is that the quantum entanglement between spins and charges is critical.
Former Rice University graduate student Xinwei Li (left) and Professor Junichiro Kono in 2016 with the terahertz spectrometer Li used to measure quantum entanglement in YbRh2Si2. (Photo by Jeff Fitlow/Rice University)
“At a magnetic quantum critical point, conventional wisdom dictates that only the spin sector will be critical,” he said. “But if the charge and spin sectors are quantum-entangled, the charge sector will end up being critical as well.”
At the time, the technology was not available to test the hypothesis, but by 2016, the situation had changed. TU Wien could grow the films, Rice had recently installed a powerful microscope that could scan them for defects, and Kono had the terahertz spectrometer to measure optical conductivity. During Bühler-Paschen’s sabbatical visit to Rice that year, she, Si, Kono and Rice microscopy expert Emilie Ringe received support to pursue the project via an Interdisciplinary Excellence Award from Rice’s newly established Creative Ventures program.
“Conceptually, it was really a dream experiment,” Si said. “Probe the charge sector at the magnetic quantum critical point to see whether it’s critical, whether it has dynamical scaling. If you don’t see anything that’s collective, that’s scaling, the critical point has to belong to some textbook type of description. But, if you see something singular, which in fact we did, then it is very direct and new evidence for the quantum entanglement nature of quantum criticality.”
Si said all the efforts that went into the study were well worth it, because the findings have far-reaching implications.
“Quantum entanglement is the basis for storage and processing of quantum information,” Si said. “At the same time, quantum criticality is believed to drive high-temperature superconductivity. So our findings suggest that the same underlying physics — quantum criticality — can lead to a platform for both quantum information and high-temperature superconductivity. When one contemplates that possibility, one cannot help but marvel at the wonder of nature.”
Si is the Harry C. and Olga K. Wiess Professor in Rice’s Department of Physics and Astronomy. Kono is a professor in Rice’s departments of Electrical and Computer Engineering, Physics and Astronomy, and Materials Science and NanoEngineering and the director of Rice’s Applied Physics Graduate Program. Ringe is now at the University of Cambridge.
Additional co-authors include Maxwell Andrews, Maximilian Bonta, Werner Schrenk, Andreas Limbeck and Gottfried Strasser, all of the TU Wien; Hermann Detz, formerly of TU Wien and currently at Brno University; Elisabeth Bianco, formerly of Rice and currently at Cornell University; Sadegh Yazdi, formerly of Rice and currently at the University of Colorado Boulder; and co-lead author Donald MacFarland, formerly of TU Wien and currently at the University at Buffalo.
The research was supported by the European Research Council, the Army Research Office, the Austrian Science Fund, the European Union’s Horizon 2020 program, the National Science Foundation, the Robert A. Welch Foundation, Los Alamos National Laboratory and Rice University.
RCQM leverages global partnerships and the strengths of more than 20 Rice research groups to address questions related to quantum materials. RCQM is supported by Rice’s offices of the provost and the vice provost for research, the Wiess School of Natural Sciences, the Brown School of Engineering, the Smalley-Curl Institute and the departments of Physics and Astronomy, Electrical and Computer Engineering, and Materials Science and NanoEngineering.
Xinwei Li, Katsumasa Yoshioka, Ming Xie, G. Timothy Noe, II, Woojoo Lee, Nicolas Marquez Peraca, Weilu Gao, Toshio Hagiwara, Ørjan S. Handegård, Li-Wei Nien, Tadaaki Nagao, Masahiro Kitajima, Hiroyuki Nojiri, Chih-Kang Shih, Allan H. MacDonald, Ikufumi Katayama, Jun Takeda, Gregory A. Fiete, and Junichiro Kono
We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of Bi1−xSbx, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition x increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed minicoil magnet, we conducted measurements in magnetic fields up to 30 T, observing distinctly different behaviors between semimetallic (x<0.07) and topological insulator (x>0.07) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blueshifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields (>20 T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi1−xSbx.
HOUSTON – (Aug. 23, 2018) – After their recent pioneering experiments to couple light and matter to an extreme degree, Rice University scientists decided to look for a similar effect in matter alone. They didn’t expect to find it so soon.
Rice physicist Junichiro Kono, graduate student Xinwei Li and their international colleagues have discovered the first example of Dicke cooperativity in a matter-matter system, a result reported in Science this week.
Rice University scientists observed Dicke cooperativity in a magnetic crystal in which two types of spins, in iron (blue arrows) and erbium (red arrows), interacted with each other. The iron spins were excited to form a wave-like object called a spin wave; the erbium spins precessing in a magnetic field (B) behaved like two-level atoms. Illustration by Xinwei Li
The discovery could help advance the understanding of spintronics and quantum magnetism, Kono said. On the spintronics side, he said the work will lead to faster information processing with lower power consumption and will contribute to the development of spin-based quantum computing. The team’s findings on quantum magnetism will lead to a deeper understanding of the phases of matter induced by many-body interactions at the atomic scale.
Instead of using light to trigger interactions in a quantum well, a system that produced new evidence of ultrastrong light-matter coupling earlier this year, the Kono lab at Rice used a magnetic field to prompt cooperativity among the spins within a crystalline compound made primarily of iron and erbium.
“This is an emerging subject in condensed matter physics,” Kono said. “There’s a long history in atomic and molecular physics of looking for the phenomenon of ultrastrong cooperative coupling. In our case, we’d already found a way to make light and condensed matter interact and hybridize, but what we’re reporting here is more exotic.”
Dicke cooperativity, named for physicist Robert Dicke, happens when incoming radiation causes a collection of atomic dipoles to couple, like gears in a motor that don’t actually touch. Dicke’s early work set the stage for the invention of lasers, the discovery of cosmic background radiation in the universe and the development of lock-in amplifiers used by scientists and engineers.
Xinwei Li, left, and Junichiro Kono of Rice University led an international effort to find the first instance of Dicke cooperativity in a matter-matter system. Photo by Jeff Fitlow
“Dicke was an unusually productive physicist,” Konosaid. “He had many high-impact papers and accomplishments in almost all areas of physics. The particular Dicke phenomenon that’s relevant to our work is related to superradiance, which he introduced in 1954. The idea is that if you have a collection of atoms, or spins, they can work together in light-matter interaction to make spontaneous emission coherent. This was a very strange idea.
“When you stimulate many atoms within a small volume, one atom produces a photon that immediately interacts with another atom in the excited state,” Kono said. “That atom produces another photon. Now you have coherent superposition of two photons.
“This happens between every pair of atoms within the volume and produces macroscopic polarization that eventually leads to a burst of coherent light called superradiance,” he said.
Taking light out of the equation meant the Kono lab had to find another way to excite the material’s dipoles, the compass-like magnetic force inherent in every atom, and prompt them to align. Because the lab is uniquely equipped for such experiments, when the test material showed up, Kono and Li were ready.
“The sample was provided by my colleague (and co-author) Shixun Cao at Shanghai University,” Kono said. Characterization tests with a small or no magnetic field performed by another co-author, Dmitry Turchinovich of the University of Duisburg-Essen, drew little response.
“But Dmitry is a good friend, and he knows we have a special experimental setup that combines terahertz spectroscopy, low temperatures and high magnetic field,” Kono said. “He was curious to know what would happen if we did the measurements.”
“Because we have some experience in this field, we got our initial data, identified some interesting details in it and thought there was something more we could explore in depth,” Li added.
“But we certainly didn’t predict this,” Kono said.
Li said that to show cooperativity, the magnetic components of the compound had to mimic the two essential ingredients in a standard light-atom coupling system where Dicke cooperativity was originally proposed: one a species of spins that can be excited into a wave-like object that simulates the light wave, and another with quantum energy levels that would shift with the applied magnetic field and simulate the atoms.
“Within a single orthoferrite compound, on one side the iron ions can be triggered to form a spin wave at a particular frequency,” Li said. “On the other side, we used the electron paramagnetic resonance of the erbium ions, which forms a two-level quantum structure that interacts with the spin wave.”
While the lab’s powerful magnet tuned the energy levels of the erbium ions, as detected by the terahertz spectroscope, it did not initially show strong interactions with the iron spin wave at room temperature. But the interactions started to appear at lower temperatures, seen in a spectroscopic measurement of coupling strength known as vacuum Rabi splitting.
Chemically doping the erbium with yttrium brought it in line with the observation and showed Dicke cooperativity in the magnetic interactions. “The way the coupling strength increased matches in an excellent manner with Dicke’s early predictions,” Li said. “But here, light is out of the picture and the coupling is matter-matter in nature.”
“The interaction we’re talking about is really atomistic,” Kono said. “We show two types of spin interacting in a single material. That’s a quantum mechanical interaction, rather than the classical mechanics we see in light-matter coupling. This opens new possibilities for not only understanding but also controlling and predicting novel phases of condensed matter.”
Co-authors of the paper are Motoaki Bamba, an associate professor at Osaka University; graduate students Ning Yuan, Maolin Xiang and Kai Xu and professors Zuanming Jin, Wei Ren and Guohong Ma at Shanghai University; Rice alumnus Qi Zhang, a research fellow at Argonne National Laboratory; and Yage Zhao, an undergraduate student at Peking University and former exchange student at Rice. Kono is a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering.
The research was supported by the National Science Foundation, the Army Research Office, the PRESTO program of the Japan Science and Technology Agency, the Japan Society for the Promotion of Science’s KAKENHI program, the ImPACT Program of the Government of Japan’s Council for Science, Technology and Innovation, the National Natural Science Foundation of China, German Research Foundation,the European Commission and the Max Planck Society.
